Varroa Mite Testing & Management

The two primary methods of sampling adult bees to estimate mite levels in a colony are the alcohol wash and powdered sugar roll.

The alcohol wash is the fastest and easiest field mite monitoring method. This lethal sampling method does not noticeably impact a populous colony. To perform the test, submerge sampled bees in alcohol, shaking and swirling them to dislodge the mites from the bees. Then, shake the varroa mites through a mesh screen and count them. Repeat the process until there are no additional varroa mites. 

A testing kit described in the brochure can be purchased from the University of Minnesota Bookstore. It is also easy to make a testing jar by using tin snips to cut size 8 hardware mesh cloth the size of the inner part of a canning jar lid. 

Link to the Newest Mite Testing Brochure (June 3, 2024)

Link to the Mite Testing Brochure in Spanish

A dish soap solution (1 tablespoon in ½ gallon of water) can be used instead of in a sampling device where a cup with a strainer sits inside of another cup. Measure 300 bees using 1/2 cup into the device that is prefilled a bit more than half-way with the detergent solution. Let the cup sit for 1 minute, then swirl the bees for at least 30 seconds. Count the mites at the bottom of the cup and dispose of the bees.

A testing kit described in the brochure can be purchased from the University of Minnesota Bookstore. It is also easy to make a testing jar by using tin snips to cut size 8 hardware mesh cloth the size of the inner part of a canning jar lid. 

The University of Nebraska developed a testing method called the “powdered sugar roll” where most of the bees in the sample can survive (unless the bees get wet or are shaken too hard). The powdered sugar roll is a test that covers sampled bees in powdered sugar to dislodge varroa mites from the adult bees. Once dislodged, varroa mites are shaken out of the bee sample through the mesh screen and counted. 

This method relies on skill developed through practice. Avoid using the powdered sugar roll in humid weather or if the bees are on a nectar flow because the powdered sugar can dissolve and become sticky, making the sample invalid. Avoid using stale or clumpy powdered sugar as it does not coat the bees well. If the shaken bees are not coated in a dusting of dry, white powdered sugar when put back into their colony, the test is invalid and the bees will likely die.

The alcohol wash mite sampling method is more reliable than the powdered sugar roll test as fewer variables affect the efficacy (e.g., humidity that results in the sugar sticking to the bees, how hard the bees are shaken). 

Learn how to do the powdered sugar roll

Ongoing research is needed to refine varroa mite thresholds. Recommended thresholds have trended lower in recent years due to the increasing impact of viruses spread by the mites. Odds of a colony dying increase as the mite pressure increases. But the reality is that thresholds can depend on local scenarios.

Higher thresholds are more possible in areas with lower viral pressure, lower density of beekeepers and colonies, avoiding bringing in stock or equipment that could introduce pathogens, and with varroa-resistant bees (lower proportion of mites in the brood). Lower thresholds may be needed in locations with high varroa mite and virus pressure.

In addition, testing adult bees for varroa mites can underestimate the true level of mites in a colony, especially when there is lots of brood. Colonies that show zero or low numbers of mites in a test can still have mite populations that grow into a level that causes problems later in the year. No detection does not mean that a colony lacks mites. As testing adult bees can underestimate a colony’s total mite burden, using a lower threshold can decrease the risk to a colony. 

Due to differences both inside a colony and out, there is no universal standard that fits every operation. A threshold can depend on a beekeeper's specific circumstances, including personal risk tolerance, historical survival rates, utilization of varroa-resistant bee stock, and if they are selecting colonies to breed varroa-resistant bees.

A conservative threshold to maintain colony health is to keep varroa mite levels low throughout the year. Aim to keep varroa levels below 1% year round (0 to 2 varroa in a 300 bee sample) for the best survival odds. A treatment may be warranted if levels are above 2%. Verify and adjust a management plan based on testing. Keep records of management practices, treatments, and mite levels.

Employ non-chemical methods to lower a colony’s varroa growth. These require a solid understanding of honey bee biology and management. Varroa testing is recommended for close monitoring of varroa populations. 

Use queens that carry varroa-resistant traits. Bee breeders can select and breed from bees that have varroa-resistant behaviors, including varroa sensitive hygiene (VSH), non-reproductive varroa, grooming, or they can select for colonies with low overall varroa growth without identifying a specific trait. 

There have been many advances in this area of beekeeping and more beekeepers are producing queens and drones that carry varroa-resistance genetics. It is not yet a guarantee that a bred queen will produce a varroa-resistant colony of bees, so be sure to still monitor varroa levels and use other management methods as needed. In addition, even colonies that exhibit behaviors associated with varroa resistance can become overwhelmed if the varroa mite or virus pressure is too high.

Use drone brood as a lure for varroa mites.

1. Secure a frame that allows the colony to rear drone brood: a foundation frame with drone-sized cells (often the frame is made of green plastic), a super frame where the bees can draw drone comb on the frame bottom, or remove the bottom two-thirds of worker brood comb on a frame.
2. In each hive box with brood, remove an empty frame or frame with honey. In a position next to the brood nest, insert the frame from step 1. Mark the frame both to find it easily and to keep using it within the same colony.
3. Return and remove the drone comb once the drone brood is capped, but before the drones emerge in about 24 days. Opening up about 100 cells with a forceps or capping scratcher can provide an estimation of the mite load.
4. To dispose of the mites and drones, cut out the drone brood or scrape it off the bottom of the super frame or freeze the drone comb for at least 24 hours. Can reuse a frozen frame and let the bees clean out the drones; in this case it can help to have a spare drone frame to put in the frame gap. Dispose of the wax away from the colony to prevent robbing.
5. Repeat the process multiple times for better efficacy. 

Considerations: 

• Must stick to a calendar to remove the frame in time and avoid mites emerging with the drones.
• Only works when the colony is rearing drone brood and drawing new wax (typically spring and summer) and is strong enough to raise drones.
• Best to use with overwintered colonies.
• There are a few steps that could increase the period of time drone larvae are available for mites to invade. One is to alternate the removal of multiple drone frames. Another is to allow the bees to build their own comb, slowing the time the queen can lay eggs in those cells. Or remove the drone brood shortly after cells are sealed; about every two weeks.
• Pairing drone trapping with a sealed brood break can result in high efficacy. 

A brood break is when there is an interruption in the queen laying eggs. No sealed brood means the varroa mites are exposed on adult bees and not reproducing under the sealed brood caps. A brood break alone is not enough to slow varroa mite growth without other management practices. 

A brood break can occur when:

• Queen is caged or removed
• Colony requeens itself
• A beekeeper requeens a colony with either a queen cell or virgin queen
• A colony is started as a package or swarm
• Queen naturally stops laying (e.g., in late fall)

Considerations: 

• Can slow colony growth, which can be the intent in order to keep a colony small. This can reduce honey production.
• Pair a brood break with a varroa treatment like oxalic acid when there is no sealed brood present for high efficacy of the treatment.

For a list of the majority of current varroa mite treatments and their labels, see the EPA-registered Pesticide Products Approved for Use Against Varroa Mites in Bee Hives. EPA registrations and pesticide labels can change over time. Follow the label on the product.

For guidance on product considerations and videos on application, see the Honey Bee Health Coalition's Varroa Management Guide. There are a few treatments that are not yet in the guide (EZ-OX, VarroxSan, Amiflex). For information on those products see this supplemental document. Additional new treatments include Api-Bioxal RTU and Norroa (Vadescana dsRNA).

To protect the applicant, it is important to read the label and wear correct personal protective equipment. At minimum, treatments require closed toe shoes, socks, long sleeves and pants, and chemical resistant gloves. Some require goggles and a respirator.

• The hardest time to control mite populations is when the colony is populous and has lots of brood as the treatment needs to get distributed throughout the colony to be effective.
• The most effective time to employ a miticide is in a broodless colony when all the varroa mites are on adult bees. Mites on adult bees are more easily killed; mites in the brood nest are difficult to get rid of because the wax cappings on the brood cells protect the mites as they reproduce underneath.
• A spring mite treatment can help reduce the likelihood of high mite levels in late summer, when large brood nests make it hard to control mite populations effectively.
• Always read the label. Follow the temperature limits, application guidelines, and wear the correct protective gear.
• To select a treatment, choose if you want a naturally-derived treatment or not (amitraz is synthetic), if there are temperature restrictions, whether the colony has brood or not, or the colony has honey supers added or will have them soon. Use the tables in the Honey Bee Health Coalition Guide or in the U of MN guidebook section on varroa to narrow treatment options down, then read the label to understand the detailed specifics for applying that product.

Below are treatment options based on colony population, when honey supers typically go on the colony, and broodless periods. However, colonies may still have issues with mites, even after using these treatments. Monitoring can help catch unexpected spikes in mite populations, allowing for adjusting a treatment plan accordingly.

Spring: Control mites in spring to limit their population growth in the summer. 

• Package: Treat with oxalic acid dribble (ApiBioxal, ApiBioxal RTU, EZ-OX) or oxalic acid vapor (ApiBioxal, EZ-OX) 5 to 7 days after installing the package. 
• Nuc: Treat with HopGuard or Norroa shortly after arrival. Or use a different miticide once the bees occupy a full deep box or two (Formic Pro or VarroxSan). Other types of treatments can be used but they must be completed before honey supers are added.
• Overwintered colony: Treat with Formic Pro, or, if mite levels are low, treat with VarroxSan or Norroa. Other types of treatments can be used but they must be completed before honey supers are added.
• Instead of, or in addition to, a treatment, use proven varroa-resistant stock and/or biology-based practices like trapping mites in drone brood.

Summer: Control mites in summer if populations spike.

• If mite levels are above 2 mites per 100 bees (6 or more mites in a 300 bee sample), use Formic Pro as it is safe to use with honey supers on. VarroxSan, also safe to use with honey supers, can be used on colonies with already low mite levels to help maintain those low levels, and if there is a box separating the honey supers from the treatment.

Before mid-August: Control mites in late summer to reduce the number of mites that damage the developing winter bees. Use a product that is effective when brood is present.

• Treat with Formic Pro, Apiguard, or ApiLife VAR. Remove supers before treatment if needed. 

Late fall: Control mites in late fall when there is no brood in the colony to remove the mites that feed on the bees over winter and to start spring with lower levels. Use a product that is effective when there is little to no brood.

• Treat with oxalic acid dribble (ApiBioxal, ApiBioxal RTU, EZ-OX) or vapor (ApiBioxal, EZ-OX) in late fall when bees are loosely clustered (about 40-50°F). Can use HopGuard, but remove the strips before winter.

Note: Oxalic acid vapor can be effective with brood present if used four times every 5 to 7 days using 4 grams of oxalic acid per box for the best efficacy (Jack et al. 2023 "Evaluating the Efficacy of Oxalic Acid Vaporization and Brood Interruption in Controlling the Honey Bee Pest Varroa destructor (Acari: Varroidae)"). However, all research was done in one deep box and it is unclear how this research translates to two- or three-deep hive boxes.